Having twin girls and so far DH and I have decided on Hannah for one of the twins but can't decide on another name!
We've tossed around
Hannah and Harper
Hannah and Kendall
Hannah and Caitlin
Hannah and Alexandra

any suggestions? We're also open to any new name ideas! also other kids are: ava, connor, adrian, and logan so we'd like the names to flow with those and last name is Grayson

Check out Nymbler.com. You can enter in your other children's names as inspiration and it will suggest names based off the sibling names. Pretty cool!
Personally, I prefer non-alliterative names for twins (no matching first letter.) Here are some I like:
Hannah and Sarah
Hannah and Lily
Hannah and Julia
Hannah and Katherine (maybe Katie for short?)

We were careful with our twin girls to pick names that sounded great when said together but were not too similar. Honestly, you might regret an alliteration. My girls names have the same theme, short and feminine, Jane Austin style names. They are Gracie and Emma. What about Hannah and Grace? (Hannah means Grace of God, that's where I got the idea!) I think Hannah and Caitlin are similar types of names and work well together. Here are some more,
Hannah and Lauren
Hannah and Sarah (if you want the theme to be Biblical)
Hannah and Emily
Hannah and Abigail (again Biblical)
Hannah and Olivia

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A crunchy wife to B and momma of identical twin girls G & E, our rainbow
baby C born at home April 2013 and R forever in my heart.